ï~~1999
THE MICHIGAN BOTANIST
53
only a few small patches of B. nigra were observed on boulders at the Gneiss
Outcrops Scientific Natural Area (SNA), where the first collection of this
species in 1997 was made. Away from the river, B. nigra was collected on glacial
erratic boulders near the summit of a boulder-strewn moraine in Yellow Medicine County (Wheeler 18580), though again only a few patches were observed.
In Pipestone and Rock counties, in extreme southwestern Minnesota, B. nigra
occurs on Sioux Quartzite, which is a metamorphosed, pinkish-red quartz sandstone that is extremely hard (Ojakangas & Matsch 1982). It is noteworthy that
the population of B. nigra at Blue Mounds State Park in Rock County is the
largest discovered to date. No collection of the species was made at Blue
Mounds State Park, but its known occurrence there is mapped in Fig. 1. The
South Dakota collection (Wheeler 18600) comes from granitic boulders atop a
boulder-strewn moraine. Further searches on boulder-strewn moraines in eastern
South Dakota would very likely yield additional records of this species.
Whereas the western portion of the Minnesota River valley is bordered primarily by igneous and metamorphic rock outcrops, the NE arm of the river is
bordered by dolomite and sandstone bluffs (Ojakangas & Matsch 1982). Scattered glacial erratic boulders do occur, however, on the remnant prairies that
border the northeastern arm of the river. Although Buellia nigra was not observed on sedimentary rocks along the NE arm of the river, the species does
occur at the Kasota Prairie in LeSueur County, where it is rare on granitic boulders. Also, B. nigra was not observed during a fortnight survey of crystalline
rock outcrops in central Minnesota (Stearns and Benton counties) in July, 1997.
Moreover, it seems to be absent from crystalline rock outcrops in northern Minnesota, a region relatively well searched for lichens.
Sheard (1969, p. 222) stated that Buellia nigra is often associated with B.
punctata (Hoffm.) Mass. [Amandinea punctata (Hoffm.) Coppins & Scheid.],
but this is not the case in south-central or southwestern Minnesota, where it is
most often associated with Aspicilia cinerea (L.) Kiber, Candelariella vittelina
(Hoffm.) Muill. Arg., Dimelaena oreina (Ach.) Norman, Lecanora muralis
(Schreber) Rabenh., Physcia subtilis Degel., Pleopsidium flavum (Bellardi)
Kirber and, less commonly, Caloplaca flavovirescens (Wulfen) Dalla Torre &
Sarnth., C. sideritis (Tuck.) Zahlbr., and Melanelia tominii (Oksner) Essl. The
brownish thallus of Buellia nigra, however, can easily be confused, at a distance,
with that of other species, such as Acarospora americana H. Magn. and A. fuscata (Schrader) Arnold. It is also worth noting that Pleopsidium flavum, which
has a bright lemon-yellow thallus and thus is one of our most distinctive lichens,
is useful as an indicator species when searching for B. nigra in southern and
western Minnesota.
Thalline patches of Buellia nigra range from (3-)10-30 mm broad (Figs. 2,
3). When present, this species typically covers less than 0.5 percent of boulder
surface area, whereas more common species, such as Aspicilia cinerea, Dimelaena oreina, and Pleopsidium flavum, can each cover as much as 25 percent or
more. But as intimated earlier, the percentage of boulder surface area covered by
B. nigra at Blue Mounds State Park, as well as at the Morton outcrops, is appreciably higher than 0.5 percent. Indeed, at the former site, on some flat, outcrop
surfaces the percentage of B. nigra cover reaches 1-3%. Relev6 data for lichen
